This study investigated the Effects of Mathematical Language on the Students’ Academic Performance in Mathematics in Public Secondary Schools in Langata Sub-County, Nairobi County. The information was acquired through the four research questions formulated based on the four objectives of this study which are: To find out the mathematics teacher’s comprehensive level of the mathematical language in public secondary schools in Langata Sub-County, Nairobi County; To investigate the frequency of use of mathematical language appropriately by mathematics’ teachers in public secondary schools in Langata Sub-County, Nairobi County; To explore on various methodologies used by mathematics teachers to enhance the learning and use of mathematical language, in public secondary schools in Langata Sub-County, Nairobi County; and To investigate the students’ attitudes towards the use of mathematical language in public secondary schools in Langata Sub-County, Nairobi County. A total of 110 questionnaires were produced and distributed to the participants of the study as follows: 104 for students and 6 for teachers. The questionnaires were administered to two public secondary Schools sampled by the researcher. The study was divided into five chapters, and each chapter was divided into sections and subsections. The first chapter outlined the general aspects of the topic under study, including a brief explanation of the statement of the problem and significance of the study. The second chapter reviewed related literature with respect to the research topic of this study. The third chapter highlighted the methodology applied by the researcher for the collection and analysis of data. the fourth chapter presented the analyzed information, and the analysis was carried out with the aid of Statistic Package for Social Science (SPSS) version 28. Lastly, the fifth chapter presented the summary, conclusions, recommendations and suggested areas for further research. The outcomes of the study evidenced that indeed mathematical language has great influence on the students’ academic performance in mathematics in the sense that, mathematics has got its own language, expressions, terminologies and symbols that are not common in the day to day life of students, and so when these are not made known to them by the teachers, it becomes a challenge for them to understand mathematics. The recommendations given by the study to the various stakeholders of the education system were: to highly consider the language of mathematics as one of the pre-requisite that mathematics’ teachers must be competent at. To frequently emphasize the usage of mathematical language as a medium of instruction by teachers and students in the teaching/learning environment, for this will make easier the learning of mathematical concepts and procedures as well as uplifting their academic performance in mathematics. Apart from using the correct methods of teaching to enhance the learning of mathematical language the study also recommended that mathematics’ teachers should always promote a friendly and active learning environment during all mathematical lessons.